The present invention relates to a sensing unit for sensing geographical displacement such as the displacement of the ground, a snow accumulation section, an underground water level or the like, and a monitoring apparatus for monitoring a landslide, a snowslide or the like using the sensing units.
There has been a pressing need for the development of a system capable of predicting the occurrence of disaster, such as a landslide caused by a heavy rain or a snowslide in a snowy area.
One conventional means for sensing the softening of the ground is such as to sense whether any of the previously stretched wires on the ground have been broken by the movement of the ground. With this type of sensing unit, however, wires have to be stretched over a wide area, which needs not only a lot of time and labor but also has the difficulty in determining the place and direction in which the ground has been displaced, resulting in a problem that it is impossible to estimate the degree of displacement.
To overcome the problems, ground sensing units using various measuring instruments have recently developed. One of them is such as to estimate the displacement of the ground, the depth of the landslide surface, and the amount of slide by use of a servo inclinometer where a weight is supported horizontally by a case via springs, or by use of a pipe strain gauge.
In the case of a ground sensing unit using the servo inclinometer, pipes are put in the holes bored in the ground and servo inclinometers are inserted into the pipes stepwise in such a manner that they can be hoisted. As the servo inclinometers are hoisted, the tilt angles are measured automatically on the basis of the displacements of the springs. By measuring the side displacement, the displacement of the ground or a continuous underground wall, i.e., a landslide can be measured.
In the case of a ground sensing unit using a pipe strain gauge, a large number of vinyl chloride pipes are inserted vertically into holes bored in the ground, while being connected to one another with intermediate pipes. Strain gauges have been stuck to suitable portions of the pipes. The resulting assemblies are fixed in place by filling the space around them with sand. By measuring the amount of bending strain while changing the strain gauge on the vinyl chloride pipe from one depth to another, the magnitude and depth of a slide can be estimated.
Such ground sensing units, however, require a large number of measuring instruments to be installed in a place where a landslide collapse may take place. Therefore, the installation work needs a lot of time and labor. The ground sensing units installed in the different positions must be connected to each other with a power cable and a communication cable.
Because the ground sensing unit using a servo inclinometer needs a moving section, it requires a large space as a whole. In the case of the ground sensing unit using pipe strain gauges, vinyl chloride pipes must be inserted vertically, while being connected to each other with intermediate pipes, and the space around the pipes must be filled with sand. Therefore, it is difficult to install a large number of pipes over a wide area in a mountainous region.
Furthermore, in the case of the ground sensing unit using the servo inclinometer or pipe strain gauge, because the side displacement of and the amount of bending strain of the ground can be measured but the position of each ground sensing unit buried in the ground cannot be sensed, they cannot be measured when all the ground has been displaced.
With this backdrop, there have been demands for a monitoring apparatus which is easy to bury in the ground and can predict the occurrence of disaster, such as a mudslide in the ground or a snowslide in a snowy region, by use of sensing units capable of sensing the displacement exactly even when the whole of the ground or the snow accumulation section has been displaced, and for a sensing unit to be used in the apparatus.
Moreover, there has been demands for a ground monitoring apparatus which is capable of sensing the displacement of the ground, regardless of the places of installed sensing units, and of predicting the occurrence of disaster, such as a mudslide in the ground, and for a sensing unit to be used in the apparatus.
As a means of predicting the occurrence of disaster, such as a mudslide in the ground, there is an underground water level sensing unit which measures the penetrating water level of rainfall in, for example, a mountainous region or a slope area or measures the position of a water vein in the ground and the state of the infiltration from the water vein. An example of the configuration of such an underground water level sensing unit is shown in FIG. 1.
In FIG. 1, numeral 21 indicates foundation concrete laid in the ground. In the foundation concrete 21, a through hole that extends from the surface of the earth into the ground is made. Numeral 22 is a cylindrical member buried in the ground in such a manner that the member is inserted into the through hole in the foundation concrete 21. The cylindrical member 22 can be adjusted so as to have a suitable length according to how deep the cylindrical member is buried. Holes that penetrate through the member are arranged in the direction of its axis.
Numeral 23 is a case placed on the foundation concrete 21, with the cylindrical member 22 in the center of the case. In the upper part of the case 23, a float driving unit 24 is provided. The float driving unit 24 holds a wire 26 in such a manner that the wire can move vertically in the cylindrical member 22. A float 25 is attached to the tip of the wire 26. The float driving unit 24 rolls up or down the wire 26 as the float 25 moves up or down according to the level of the water accumulated at the bottom of the cylindrical member 22.
In the lower part of the case 23, a measuring instrument 27 and a transmitter 29 are provided. The measuring instrument 27 measures the level of the water accumulated at the bottom of the cylindrical member 22 from the movement of the wire 26 rolled up or down by the float driving unit 24. The transmitter 29 transmits the data measured at the measuring instrument 27 to a base station (not shown) via an output cable 28 laid in the ground.
In the underground water level sensing unit constructed as described above, when rainwater has permeated into the ground, the water passes through the holes arranged in the direction of the axis of the cylindrical member 22 and collects at the bottom of the cylindrical member 22. The level of the water accumulated at the bottom of the cylindrical member 22 is measured by the measuring instrument 27 from the movement of the wire 26 rolled up or down according to the up-and-down movement of the float 25.
Such an underground water level sensing unit, however, can measure only the water accumulated at the bottom of the cylindrical member 22 but cannot judge how much the water has come from how depths of the stratums.
In a landslide danger zone, such as a slope area, it is important to measure how much rainwater has permeated into the ground and judge whether the rainwater has reached a stratum that is liable to cause a landslide. According to the conventional float-type water level sensing unit, however, the water which has come from all of an upper stratum, an intermediate stratum and a lower stratum is accumulated at the bottom, so that it is impossible to judge how much the water has come from how depths of the stratums, which prevents effective prediction of a landslide.
The float-type water level sensing unit has another problem: when the cylindrical member 22 has been deformed and the float 25 has come into contact with the inner wall of the cylindrical member 22, this prevents the float 25 from making up-and-down movement and makes it difficult to accurately measure the water level.
Moreover, in installation, a hole must be bored vertically with high accuracy so that the float 25 may not touch the inner wall. To achieve this is difficult when the hole is as deep as several tens of meters.
Thus, there have been demands for an underground water level sensing unit capable of measuring the position and size of a very wet stratum in the ground.
As for a means of sensing a snowslide, props have been placed at suitable intervals in a place where a snowslide is liable to take place. A resistive wire has been stretched at a suitable height between the props and normally been made conducting. A change in the resistance value caused by the breaking of the wire due to a snowslide is sensed. The value has been transmitted by cable to a base station, which then senses the occurrence of a snowslide.
Such a snowslide monitoring system, however, requires wire to be laid over a wide area. This needs not only a lot of time and labor but also has the difficulty in pinpointing the place where the snowslide has occurred, resulting in a problem that it is impossible to predict the size of the snowslide.
If the movement of a snow accumulation layer imposed a heavy load on the props and they were bent, the props would suffer damage or remain bent. Therefore, it is necessary for maintenance personnel to repair or replace the props after the snow has melted.
Furthermore, since each prop has not been installed on a foundation, there occurs a problem in which the props, together with the snow, might be carried away if a snowslide took place.
Thus, there have been demands for a snowslide monitoring apparatus capable of not only determining the place where a snowslide has occurred and its size but also automatically restoring the sensing units to their original position after the snow has melted even when the sensing units have been bent under the weight of snow or by the movement of snow, and for a sensing unit to be used in the apparatus.